GB2138103A - Skirt structure for trunk piston - Google Patents

Abstract

A piston casting for forming a piston for an internal combustion engine or compressor comprises a generally cylindrical crown (10) having a ring belt (11) extending around the crown for the formation therein of one or more piston ring grooves. A gudgeon pin bore (14) extends through the piston casting in a direction normal to the piston axis and two opposed skirt thrust surfaces (16) depend from the ring belt and are symetrically disposed about a plane including the piston axis and normal to the gudgeon pin bore axis. The surfaces extend only part the way around the piston casting and terminate in respective spaced generally axially extending edges (18), each skirt thrust surface being extended circumferentially beyond the associated edges (at 19) at the ends of the edges remote from the crown and/or the piston casting comprising two skirt side panels (17) extending between the skirt thrust surfaces in planes normal to the gudgeon pin axis separated by a distance which is 0.6-0.8 times the diameter of the skirt thrust surfaces. <IMAGE>

Description

SPECIFICATION Pistons and piston castings The invention relates to piston castings for forming a piston for an internal combustion engine or compressor and to the pistons themselves. In particular, the invention relates to such piston castings and pistons of the kind having opposed part-cylindrical thrust sur faces.

According to a first aspect of the invention, there is provided a piston casting for forming a piston for an internal combustion engine or compressor and comprising a generally cylin drical crown having a ring belt extending around the crown for the formation therein of one or more piston ring grooves, a gudgeon pin bore extending through the piston casting in a direction normal to the piston axis, two opposed skirt thrust surfaces depending from the ring belt and symetrically disposed about a plane including the piston axis and normal to the gudgeon pin bore axis, the surfaces extending only part the way around the piston casting and terminating in respective spaced generally axially extending edges, each skirt thrust surface being extended circumferentially beyond the associated edges at the ends of the edges remote from the crown.

According to a second aspect of the invention, there is provided a piston for an internal combustion engine or compressor and comprising a generally cylindrical crown having a ring belt extending around the crown and including one or more piston ring grooves formed therein, a gudgeon pin bore extending through the piston and having an axis normal to the piston axis, a skirt depending from the ring belt and extending around the piston, the skirt forming two opposed arcuate skirt thrust surfaces symetrically disposed about a plane including the piston axis and normal to the gudgeon pin bore axis, and two skirt side panels extending between the skirt thrust surfaces and lying in respective planes normal to the gudgeon pin bore axis, the spacing of the side panels being between 0.6 and 0.8 of the diameter of the skirt thrust surfaces.

The following is a more detailed description of two embodiments of the invention, by way of example, reference being made to the accompanying drawings, in which: Figure 1 is an elevation of a piston casting, Figure 2 is a vertical cross-section through the piston casting of Figure 1, Figure 3 is a vertical cross-section of the casting of Figures 1 and 2 in a plane at right angles to the sectional plane of Figure 2, Figure 4 is a section on the lines IV-IV of Figure 3, Figure 5 is a section on the line V-V of Figure 2, and Figure 6 is a similar view to Figure 1 but showing a piston formed from a piston casting of the kind shown in Figures 1 to 5 but modified by the provision of holes or windows in side panels of the piston casting.

Referring first to Figure 1, the piston cast ing is made from aluminium or any suitable aluminium alloy, such as that sold by the trade name Lo-Ex. The casting may be formed conventionally or by a squeeze casting pro cess or by any other suitable casting process.

The casting comprises a generally cylindri cal crown 10 having a ring belt 11 extending around the crown 10 for the formation therein of one or more piston ring grooves, There may be three grooves. Two gudgeon pin bosses 1 2 are connected to the crown by a web 1 3 and define a gudgeon pin bore 14 whose axis is normal to the piston axis. Although only one web 1 3 is shown, there may be two or more such webs. An expansion control insert 1 5 is cast into the casting and may be of any suitable kind such as that described in our copending British Patent Application No. 80.30430.

A skirt depends from the ring belt 11 and includes two opposed thrust surfaces 1 6 and two side panels 1 7. The thrust surfaces 1 6 are arcuate and are symetrically disposed about a plane including the piston axis but normal to the gudgeon pin bore axis. These thrust surfaces 1 6 extend only part the way around the casting, generally for an arcuate extent of 120 although this figure may be greater or smaller. Each thrust surface 1 6 terminates in spaced axially extending edges 1 8 (see Figs. 1 and 5). At the end of each edge 18 remote from the crown 10, the thrust surface is continued to form a projection 19.

The side panels 1 7 extend between adjacent edges 18 of the thrust surfaces 1 6 and around the entrances to the gudgeon pin bore 14. The panels 1 7 lie in respective planes parallel to, but spaced on either side of, the piston axis. The panels 1 7 are not connected directly to the edges 18 of the thrust surfaces 16; bevelled surfaces 20 are provided between them. The spacing between the side panels 1 7 is between 0.6 and 0.8 of the diameter across the thrust surfaces 16. A preferred ratio is 0.7.

Two pairs of slots 21 are also provided in the casting. The slots 21 are so positioned that when the lowermost piston ring groove is machined in the belt 11, the slots provide oil drainage slots leading from a wall of the groove remote from the crown. The pairs of slots 21 are arranged in respective parts of the ring belt which as seen in Fig.2, overhang the side panels 1 7.

After casting, the piston casting is finish machined. The piston ring grooves are cut, with the slots 21 then forming drainage slots.

The drainage slots 21, being cast in, eliminate the need for drilling through the oil ring groove to give direct drainage to the inside of the piston and hence to the sump. The external drainage provided by the slots 21 is advantageous when piston cooling by oil jet is used since it eliminates any outflow of oil through drain holes dribbled from the oil ring groove. With the external drainage slots 21, the presence of relatively large amounts of oil does not interfere with the proper function of the oil scraping ring, which, experience shows, does require some drainage.

The thrust surfaces 1 6 are also machined with the bevelled surfaces 20 assisting machining and preventing burrs, which are commonly formed when the machining operation is a diamond turning. Rounded corners are also provided at the outer ends of the slots 21 for the same purpose. The projections 1 9 also assist the finish machining of the casting by extending the circumferential length of the lower edge of the skirt so providing a better platform for steadying the piston while it is being machined. In addition, the projections 1 9 also eliminate undesirable stress raisers.

It will be appreciated that the features of the projections 19, the bevelled surfaces 20 and the slots 21 need not all be provided together on a piston casting, they may be provided separately or in any desired combination. The piston so formed is light in weight yet has required rigidity/flexibility characteristics and this is due to the ratio of the distance between the side panels 1 7 and the diameter across the thrust surfaces 1 6. If this ratio is less than 0.6, the skirt is very rigid and 'box-like" so that the required rigidity/flexibility in the thrust surfaces 16 can only be achieved by cutting slots in the skirt, and this is undesirable. If the ratio is greater than 0.8, the cross-sectional shape of the skirt is too close to the conventional circular crosssection shape to afford any weight saving.

Figure 6 shows a finished piston formed from a piston casting of the kind described above with reference to Figures 1 to 5. Parts common to Figures 1 to 5 and to Figure 6 are given the same reference numerals and are not described in detail. In the casting from which the piston of Figure 6 is formed, the side panels 1 7 are provided with holes or windows 22. There are two such holes or windows 22 on each panel, one on each side of the entrance to the gudgeon pin bore 14.

The holes or windows 22 may have any convenient shape or size, such as those shown in broken and chain dotted lines in Figure 6. There may be any convenient number of holes or windows. The purpose of the holes or windows 22 is to reduce the weight of the piston, Since the side panels 1 7 are relatively lightly stressed, the provision of such holes or windows 22 has no substantial effect on the strength of the piston.

The piston includes three piston ring grooves 23, and it will be seen that the lowermost piston ring groove cuts through the slots 21 so that the slots 21 form passages leading from the lowermost radial surface of the lowermost piston ring groove to respective points above the side panels 1 7. This allows the drainage of oil from the piston ring groove, as described above.

Claims (11)

1. A Piston casting for forming a piston for an internal combustion engine of compressor and comprising a generally cylindrical crown having a ring belt extending around the crown for the formation therein of one or more piston ring grooves, a gudgeon pin bore extending through the piston casting in a direction normal to the piston axis, two opposed skirt thrust surfaces depending from the ring belt and symmetrically disposed about a plane including the piston axis and normal to the gudgeon pin bore axis, the surfaces extending only part the way around the piston casting and terminating in respective spaced generally extending edges, each skirt thrust surface being extended circumferentially beyond the associated edges at the ends of the edges remote from the crown.

2. A piston casting according to claim 1, and further including skirt side Panels extending between adjacent edges of the skirt thrust surfaces, the side panels Iying in respective planes parallel to, but spaced on either side of, the piston axis.

3. A piston casting according to claim 2, wherein each side panel is connected to the associated thrust surfaces by bevelled surfaces extending between the edges of the thrust surfaces and the side panels.

4. A piston casting according to any one of claims 1 to 3, wherein each side panel includes at least one aperture, for reducing the weight of the finished piston.

5. A piston casting according to any one of claims to 4 and further including at least one slot so positioned in the ring belt that, on machining of a piston ring groove in the belt, the at least one slot provides an oil drainage slot leading from a wall of the piston ring groove remote from the crown.

6. A piston casting according to claim 5, wherein two pairs of such slots are provided, the slots of each pair being at locations between the two skirt thrust surfaces, in a circumferential direction.

7. A piston casting substantially as hereinbefore described with reference to Figures 1 to 5 or to Figures 1 to 5 as modified by Figure 6 of the accompanying drawings.

8. A piston for an internal combustion engine formed from a piston casting according to any one of clairns 1 to 7.

9. A piston for an internal combustion engine or compressor and comprising a generally cylindrical crown having a ring belt extending around the crown and including one or more piston ring grooves formed therein, a gudgeon pin bore extending through the piston and having an axis normal to the piston axis, a skirt depending from the ring belt and extending around the piston, the skirt forming two opposed arcuate skirt thrust surfaces symmetrically disposed about a plane including the piston axis and normal to the gudgeon pin bore axis, and two skirt side panels extending between the skirt thrust surfaces and lying in respective planes normal to the gudgeon pin bore axis, the spacing of the side panels being between 0.6 and 0.8 of the diameter of the skirt thrust surfaces.

10. A piston according to claim 9, wherein each side panel is connected to the associated thrust surfaces by bevelled surfaces extending between the edges of the thrust surfaces and the side panels.

11. A piston according to claim 9 or claim 10, wherein each side panel includes at least one aperture, for reducing the weight of the piston.

1 2. A piston according to any one of claims 9 to 11 and further including at least one slot in the lowermost piston ring groove and leading from a wall of said piston ring groove remote from the crown, the at least one slot being located above a side panel of the piston.

1 3. A piston according to claim 12, wherein two pairs of such slots are provided, the slots of each pair being located above an associated one of the side panels.

1 4. A piston substantially as hereinbefore described with reference to the accompanying drawings.